In recent decades, water consumption has globally increased sharply as a consequence of rapid world population growth, along with accelerated industrialization and urbanization. Also, discharge rates of various domestic sewage, industrial wastewater, and livestock wastewater have increased, thereby contaminating various streams and lakes. Stream contamination may destroy the ecosystem and also may increase water purification costs due to the deterioration of the quality of different water sources and may also paralyze some of water treatment facilities.
On the other hand, while 70% of the surface of the globe is covered by water, fresh water represents only 2% of the total global water supply of which 90% is not usable as it is in the form of ice in the poles. Due to this limitation, it is necessary to seriously consider water reuse (recycling) and optimization of its consumption to be able to partially compensate for this limitation. On the other hand, most of the consumed water is converted into sewage, having a severely negative impact on the environment and human health due to sewage containing industrial toxins which cause deterioration of underground and surface water quality by entering underground water tables, rivers, and seas.
The measures taken so far to address the issues related to water treatment may include, use of screens or basins for settling suspended particles in water, use of sand (gravel) filters, use of chemical coagulants, or pressure systems, etc. The use of settling basins requires constructing multiple basins entailing using a vast area of land, leading it them to be costly and time consuming to construct.
Slow sand filters are known to be effective in removing bacteria, parasites, Giardia cysts, cryptosporidium oocysts, and viruses. Slow sand filters differ from all other filters used to treat drinking water in that they work by using a complex biological film that grows naturally on the surface of the sand. The sand itself does not perform any filtration function but simply acts as a substrate, unlike its counterparts for UV and pressurized treatments. Although they are often preferred technology in many developing countries because of their low energy requirements and robust performance, they are also used to treat water in some developed countries While bacteria, parasites, may be killed or rendered inactive with disinfection in slow sand filters, dormant cysts and oocysts are very resistant to disinfection unless a respective disinfectant is administered very intensively. Slow sand filters may be used to supply clean water directly for consumption or may be used to supply water for further treatment, such as by reverse osmosis and ultra-filtration, both of which processes require relatively clean water to begin with. In other words, reverse osmosis and ultra-filtration are not able to filter significantly infected water.
A slow sand filter is, generally, able to remove a small amount of ammonium, manganese, bacteria, and odorant from the raw water through the biological decomposition. However, the slow sand filter is, generally, not able to remove large amounts of the contaminants because it does not utilize coagulation and flocculation processes. On the other hand, in a rapid sand filter system that may be used to substitute a slow sand filter, suspended materials in the raw water may be agglomerated in a flocculation process, settled in a sedimentation basin and sieved through a granular media consisting of sand and gravel. Although the performance of a rapid sand filter is better than a slow sand filter for most water sources, it may also be associated with some challenges based on concentration and species of the contaminants in the raw water. Especially when algae bloom due to the eutrophication of water in a reservoir or in a stagnated stream, disinfection function of the rapid sand filter may attenuate rapidly due to the high volume of contaminants.
Sand filters (also called gravel filters) generally need pre-treatment and they are not efficient for waters with a high concentration in terms of abundance of suspended particles. During washing of a sand filter, the water disinfection operation is paused and the sand filters are being washed and large amounts of water is consumed for washing the filter.
In high-pressure filtration systems, the use of hydraulic pressure to increase discharge rate leads to finer particles or increased dissolution of particles inside water, thus increasing and complicating the treatment process, and subsequently leading to increase in treatment costs. Therefore, there is a need for a simple and low-cost system for disinfecting water.